We plan to test the hypothesis that gamma-delta T cells play an important role in tolerance to exogenous and endogenous antigens. This idea is based on our experiments determining how CD8+ T cells down regulate immune responses. We have shown that exogenous antigens, such as ovalbumin (OVA) mixed with complete Freund's adjuvant (CFA) primes OVA-specific, MHC class I restricted, CD8+ CTL. Transfer of these CTL to naive, syngeneic recipients inhibited OVA-specific responses suggesting that CTL could account for some of the suppressor activities of CD8+ T cells. To test whether CTL were involved in a typical model involving suppression, oral tolerance to OVA was studied. We found that oral administration of OVA did not prime CTL but rather inhibited the priming of CTL. in addition, CD8+ splenic T cells from tolerant mice transferred tolerance to naive mice suggesting that not all suppressors are CTL. Moreover, pretreatment of mice with anti-gamma-delta TcR antibody prevented tolerance induction by OVA and reversed self tolerance to insulin. Indirect evidence suggests that anti-gamma-delta antibody interferes with tolerance induction by depleting gamma-delta T cells. Also, oral administration of OVA failed to induce tolerance in gamma-delta knock out mice providing additional support for the interpretation that gamma-delta T cells play an essential role in tolerance induction. The goal of this project is to determine the role of gamma-delta T cells in tolerance using OVA as an extrinsic or an intrinsic antigen with particular emphasis on OVA as a tumor antigen. We will test whether the CD8+ suppressor T cells induced by oral administration of OVA are gamma- delta T cells or whether gamma-delta T cells regulate the activation of CD8+ suppressor T cells. We will determine whether gamma-delta T cells are OVA-specific and produce T cell clones or hybridomas to study their receptors. The role of gamma-delta T cells in self-tolerance also will be studied using transgenic (Tg) mice expressing the OVA gene. We will determine whether these mice are tolerant to OVA in CFA and whether anti- gamma-delta antibody reverses tolerance. Tg mice will be given oral OVA to determine whether gamma-delta T cells and/or CD8+ suppressor T cells are tolerant. The possibility that gamma-delta T cells could be involved in the failure of tumors to be rejected will be tested using EL4 cells expressing the OVA gene as a tumor model. The effect of tolerance or immunity to OVA on tumor rejection will be evaluated and the effect of anti-gamma-delta antibody on tumor rejection will be assessed in normal and OVA Tg mice. The ability of tumors transfected with co-stimulators to overcome tolerance and induce rejection will be compared in normal and Tg mice.